Liquid metal ion source

ABSTRACT

This invention relates to a liquid metal ion source which melts a source material and extracts ions. Stable extraction of ions of at least one element selected from among As, P and B for a long period of time can be attained by using as a source material an alloy having a composition represented by the formula L X  R Y  M A  wherein X, Y and A each stands for atomic percentage; L at least one element selected from among Pt, Pd and Ag; R at least one element selected from among As, P and B; M at least one element selected from among Ge, Si and Sb; 5&lt;A&lt;50; 40&lt;X&lt;70; and X+Y+A=100.

TECHNICAL FIELD

This invention relates to a liquid metal ion source suitable as an ionsource for a maskless ion implanter, a micro-zone secondary ion massspectrometer, a micro-zone deposition apparatus or the like. Moreparticularly, the present invention is concerned with a liquid metal ionsource suitable for stably extracting ions of at least one elementselected from the group consisting of boron (B), phosphorus (P) andarsenic (As) for a long period of time.

BACKGROUND ART

In recent years, a liquid metal ion source has attracted attention,because it can emit an ion beam having a high brightness and a finediameter of the order of submicrons, which provide a possibility thatlithography, doping (implantation), etching, etc. involved insemiconductor processes can be conducted without the use of any mask(i.e., by the maskless method) which has conventionally been required orwithout resort to any chemical means.

The liquid metal ion source operates according to the followingprinciple. First, a source material (liquid metal) which has been meltedby means of resistance heating, electron bombardment, laser radiation orthe like is fed to an emitter made of a high-melting material such astungsten (W), molybdenum (Mo), tantalum (Ta) or silicon carbide (SiC)and having a sharply pointed tip. Application of a negative high voltageto an extraction electrode brings about concentration of an electricfield at the tip of the emitter. When a high voltage is further appliedand reaches a certain threshold value, the liquid metal located at thetip of the emitter forms a conical protrusion called Taylor Cone,leading to an extraction of ions from the tip.

When such a liquid metal ion source is intended for use in variousfields, it is an important requisite that the liquid metal ion sourcecan stably emit an intended ion beam for a long period of time.

Meanwhile, among n-type impurities for silicon semiconductors, the mostimportant elements are arsenic (As) and phosphorus (P) while boron (B)is important with respect to p-type impurities. Phosphorus in the formof a simple substance has a melting point of 44.1° C., and the vaporpressure of P₄ at that temperature is as high as about 24 Pa, whichmakes it difficult to use phosphorus in the form of a simple substanceas a source material for a liquid metal ion source. Similarly, arsenicin the form of a simple substance cannot be used as a ion source,because arsenic in the form of a simple substance has a melting point of817° C. while its vapor pressure at that temperature is as high as3.6×10⁶ Pa. Further, boron in the form of a simple substance is alsounsuited as a source material because of its high melting point of about2400° C.

When an element in a simple substance form which emits an intended ionhas a high vapor pressure or a high melting point as mentioned above,the intended element must be converted into an alloy or compound incombination with other elements in order to reduce the above-mentioneddifficulties, and the alloy or compound is used as a source material.When the alloy or compound is used as a source material, the emittedions contain ions of other elements and ions of molecules in combinationwith other elements besides the intended ion. In such a case, aneffectively employed method is one in which a mass spectrometer isprovided after the ion source to obtain only the intended ion. In fact,such a method has often been used conventionally. For example, whenemission of silicon (Si) ions from a liquid metal ion source isintended, silicon is used as the source material not in the form of asimple substance having a melting point of about 1420° C. but in theform of an alloy thereof with gold (Au), i.e., Au-Si. The melting pointof the alloy Au-Si in a eutectic composition form is about 370° C.,i.e., much lower than that of silicon. The lowering in melting pointadvantageously contributes to reduction in electric power consumedduring melting as well as reduction in frequency of heat damage to aheater or emitter and prevention of excessive evaporation of the sourcematerial.

With respect to extraction of As ions from a liquid metal ion source,the following source materials have been proposed: Sn₆₈ Pb₂₄ As₈reported by Gamo et al. in Jpn. J. Appl. Phys. Vol. 19, No. 10 (Oct.,1980) L. 595 to 598 entitled "B, As and Si Field Ion Sources"; Pd₄₀ Ni₄₀B₁₀ As₁₀ reported by Wang et al. in J. Vac. Sci. Technol., Vol. 19, No.4 Nov./Dec., 1158-1163 (1981) entitled "A mass-separatingfocused-ion-beam system for maskless ion implantation"; and a Pt-Asalloy reported by Shiokawa et al. in J. Vac. Sci. Technol. B, Vol. 1,No. 4, Oct.-Dec. 1, 1117-1120 (1983) entitled "100 keV focused ion beamsystem with an E×B mass filter for maskless ion implantation."

Only one article on extraction of P ions from a liquid metal ion sourcehas been reported by Ishitani et al. in Jpn. J. Appl. Phys., Vol. 23(1984) L 330-332 entitled "Development of Phosphorus Liquid-Metal IonSource." In this report, an alloy of copper with phosphorus, i.e., Cu₃ P(P concentration: 25% in terms of the number of atoms) is used as thesource material. This report describes that, among emitted ions, P⁺ hasthe highest intensity, and P²⁺ has the second highest intensity withrespect to phosphorus ions.

Further, with respect to extraction of B ions from a liquid metal ionsource, there is an article reported by Ishitani et al. in Nucl.Instrum. & Methods, Vol. 218, 363-367 (1983) entitled "Mass-separatedMicrobeam System with a Liquid-Metal-Ion-Source." It is not favorable touse metallic materials as materials for an emitter or a heater(reservoir), because boron easily reacts with a metal at a hightemperature, leading to a short service life of the ion source. However,in the above-mentioned fifth conventional ion source, an ion source lifeof 200 hr is attained by using an alloy (melting point: about 1000° C.)represented by the formula Ni₅₀ B₅₀ as the source material and using anemitter made of a carbonaceous material called glassy carbon.

The above-mentioned conventional ion sources had the following problems.It is reported that mass analysis of ions emitted by using Sn₆₈ Pb₂₄ As₈as the source material revealed that the amount of the emitted As⁺ ionswas as small as 0.4% based on the total of the emitted ions, that ofAs²⁺ was 0.1% and As³⁺ was 0.1% and that the service life was about 5hr. As to a Pt-As alloy, it is reported that the life of the ion sourcewas about 10 hr. With respect to the use of CuP₃ as the source material,an apparatus mounting an ion source which uses this source materialneeds provision of a high-resolution mass spectrometer having a massresolution of at least 63, because the mass/electric charge ratio, i.e.,m/e (m: mass number; e: electric charge number) of P⁺ is 31 while thatof a divalent Cu ion, i.e., ⁶³ Cu²⁺ which is the other one of theelements constituting the source material is 31.5, i.e., the differencein m/e between the two elements is as small as 0.5. Further, it isreported that the service life of the ion source was about 20 hr. Theboron ion source proposed by Ishitani et al. which uses an emitter madeof a glassy carbon involves a problem that the source materialscontaining elements capable of emitting intended ions are limited inkind, because metals wettable with a carbonaceous material such as aglassy carbon is limited in kind, e.g., Ni is easily wetted while Pt,Cu, Pd, etc. are difficultly wetted.

As mentioned above, the prior art had various problems such as a shortservice life of ion sources and a small amount of ionic current withrespect to As and P ion sources; and, with respect to B ions, a limitedkind of source materials usable for emitting B ions due to a limitedkind of metals wettable with a carbonaceous material which has been usedfor avoiding a reaction between B and the metal. Hence, in the priorart, the liquid metal ion source has not satisfactorily been applied forstably extracting As, P or B ions for a long period of time and forimplanting the extracted ions into a Si semiconductor substrate.

In view of the above situations, there has been desired to develop aliquid metal ion source capable of stably emitting As ions, P ions or Bions, or ions of at least one kind of element out of these three kindsof element for a long period of time by making use of a source materialwhich is relatively low in melting point, sufficiently wettable with theemitter, reservoir or heater, small in degree of selective evaporationof As or P and undergoes no significant change in melting pointattributable thereto.

DISCLOSURE OF INVENTION

The present invention has been made under these circumstances, and anobject of the present invention is to provide a liquid metal ion sourcefrom which ions of at least one element selected from among As, P and Bcan stably be extracted for a long period of time.

The above object can be attained by a liquid metal ion source comprisinga reservoir which contains a source material in a molten state, anemitter which has been arranged so that said molten source material fedfrom said reservoir is emitted in the form of ions from the tip thereofand an extracting electrode which serves to extract ions from the tip ofsaid emitter, characterized in that said source material is an alloyhaving a composition represented by the formula L_(X) R_(Y) M_(A)wherein X, Y and A each stands for atomic percentage; L at least oneelement selected from among Pt, Pd and Ag; R at least one elementselected from among As, P and B; M at least one element selected fromamong Ge, Si and Sb; 5<A<50; 40<X<70; and X+Y+A=100.

It is preferred that the source material comprises an alloy having acomposition represented by the formula L_(X) R_(Y) M_(A), wherein Lstands for at least one element selected from Pd and Pt; R at least oneelement selected from As and P; 5<A<50; 40<X<70; and X+Y+A=100.

In order to stably obtain large amounts of ionic currents of As and Pfor which the extraction of ions from a liquid metal ion source bymaking use of an element in the form of a simple substance has beenconsidered to be difficult due to their high vapor pressures althoughthe ions thereof are regarded as important ones in Si semiconductorprocesses, the present inventors have attempted to develop a liquidmetal ion source which uses an alloy containing As or P having arelatively low melting point and exhibiting a low vapor pressure whenmelted, which can provide emission of As⁺ and As²⁺, or As²⁺ and P²⁺ ofwhich the m/e ratios are not near those of the other elements and whichcan make it possible to separate intended ions even by means of a massspectrometer having a resolution of about 30 and to obtain As⁺ and As²⁺,or P⁺ and P²⁺ in the form of a simple substance ion beam, and arrived atthe present invention.

The present inventors first attempted to extract As ions, P ions and Bions respectively from three alloys, i.e., an alloy of the formula Ag₇₅As₂₅ (melting point: about 540° C.), an alloy of the formula Pt₈₀ P₂₀(melting point: about 590° C.) and an alloy of the formula Pt₆₀ B₄₀(melting point: about 830° C.). However, there arose the followingproblems associated with the above three alloys.

Ag-As alloy and Pt-P alloy

Vigorous selective evaporation of As and P from the both molten alloys(liquid metals) occurred, which caused changes in composition ratios inAg-As and Pt-P with time, leading to elevation of the melting point andfinally resulting in a problem that the emission of As ions and P ionsstopped 10 hr after the initiation of the emission. This is attributableto the high melting points of As and P.

With a view to suppressing the elevation of the melting points (i.e.,selective evaporation of As or P) of the Ag-As alloy and Pt-P alloy anddeveloping a source material capable of continuously emitting As ions orP ions for a long period of time, the elements Ag, As and Ge were mixedto prepare a ternary alloy having an atomic composition of the formulaAg₆₀ As₃₂ Ge₈. With respect to the P ion source, a ternary alloy havinga composition of the formula Pt₆₈ P₁₇ Sb₁₅ was prepared from Pt, P andSb. These alloys were mounted respectively on ion sources and melted toemit ions. As a result, it was found that both the alloys had a meltingpoint of about 700° to 800° C. and continuously emitted As ions or Pions without causing any significant rise in the melting point evenabout 100 hr after initiation of emission of the ions. This is becausethe third elements, i.e., Ge or Sb which has been added to the Ag-Asalloy or Pt-P alloy served to suppress selective evaporation of As or P,so that the melting points were stably kept for a long period of time.The amount of Sb or Ge to be added is preferably more than 5 atomic %.When the amount of the element is less than the above range, the addedelement does not sufficiently suppress the rise of the melting point. Onthe other hand, when Sb or Ge is added in an excessive amount andaccounts for the major part of the above-mentioned ternary alloy, theamount of ionic current of intended As ions or P ions is remarkablydecreased, causing lowering in practicality thereof for use as a sourcematerial for emission of As ions or P ions. Therefore, it is preferredthat the amount of the third element, i.e., Sb or Ge, to be added be atmost 50 atomic % based on the total.

The above-mentioned effect with respect to the suppression of the riseof the melting point attributable to addition of Sb or Ge can also beattained by addition of Si, and the addition of at least one elementselected from among Sb, Ge and Si to the above Ag-As alloy or Pt-P alloyproduces an effect with respect to suppression of the rise of themelting point. Further, a similar effect can also be attained when themetals constituting the matrix comprise a combination of As with Pt orPd besides Ag and a combination of P with Ag or Pd besides Pt.

In this connection, it is to be noted that the elements Si, Sb and Ge tobe added to the above-mentioned binary alloys, i.e., Ag-As, Pt-As,Pd-As, Ag-P, Pt-P and Pd-P, cannot easily be found out based on onlyknown physical data such as the periodic table, constitutional diagram(phase diagram) of alloys and melting points. In other words, eventhough it can be expected that the addition of a third or fourth elementto an alloy temporarily serves to lower the melting point of the alloy,whether or not the alloy to which the element has been added cansatisfactory be used as a source material which is mounted on an ionsource cannot be determined based on such expectation. This is becauseother important requisites should also be taken into account, such asnecessity of suppressing selective evaporation for maintaining themelting point and composition of a liquid metal at a constant value fora long period of time and for stably feeding the liquid metal to anemitter at its tip to cause ionization of the metal and necessity of nooccurrence of a reaction between the liquid metal and the emitter orreservoir for the source material. An alloy comprising, in combination,at least one matrix metal selected from among Pt, Pd and Ag, at leastone intended element selected from As and P and at least one element tobe added for suppression of the rise of the melting point selected fromamong Si, Sb and Ge can satisfactorily meet such strict requisites.

Pt-B alloy

As mentioned above, since B reacts chemically with metallic materials athigh temperatures with great ease, tungsten (W) and molybdenum (Mo)which have been used for conventional liquid metal ion sources cannot beused as an emitter or reservoir material for an ion source. To solvesuch a problem, a carbonaceous material has been used for the emitter orreservoir to prevent the reaction with B. Since a molten Ni is highlywettable with a carbon aceous material, a Ni-B alloy has been used as asource material for B ions.

Thus, although the carbonaceous material exhibits an excellent effectwith respect to suppression of the reaction with B, metals wettable withthe carbonaceous material are limited in kind. A typical example of sucha wettable metal is Ni. However, a source material comprising Ni as thematrix metal has the following drawbacks.

The effectiveness of emission of ions of an n-type dopant and a p-typedopant for a semiconductor substrate which is a target of ionimplantation from one ion source can easily be understood from the factthat n-type ions and p-type ions can be selectively implanted by simplyadjusting a mass spectrometer provided after the ion source. The n-typedopant and p-type dopant, e.g., for a Si substrate include As, P and Sbfor n-type and B for p-type. If extraction of both the B (p-type) ionsand P (n-type) ions from one ion source is desired as mentioned above,an emitter or reservoir made of a carbonaceous material shouldnecessarily be used in view of the reactivity between B and the metals.Since Ni is easily wettable with the carbonaceous material, it isexpected that a Ni-B-P alloy would be suitable as a source material forextracting both the B ions and P ions. However, when such an alloy isemployed as the source material, ⁶² N²⁺ and ³¹ P⁺ cannot be separatedfrom each other by means of a mass spectrometer, which makes itimpossible to obtain a simple element ion beam consisting of P⁺. This isattributable to the fact that the mass/electric charge ratio, i.e., m/e(m: mass; e: number of electric charges) of ⁶² Ni²⁺ is the same as thatof ³¹ P⁺ and is 31. Thus, the use of Ni as the matrix metal has a fataldisadvantage that it cannot be used in combination with P.

The B-containing alloys other than the Ni-B alloy include Pt-B alloy andPd-B alloy. However, since the both alloys are not wettable with acarbonaceous material at all, they cannot serve as an ion source at all.Even when the emitter or reservoir made of a metal is used, the servicelife of the ion source is as short as several hours. This is a fataldrawback.

The present inventors have made studies on a variety of elements to beadded as the third or fourth element to the Pt-B alloy with a view toimproving the wettability of the alloy with the carbonaceous material.As a result, the present inventors have found that Sb, Si and Ge areeffective as the additive. For example, the comparison in terms of theion source life showed that when a Pt-B alloy is used the ion sourcelife is as short as several hours, while a Pt-B-Si ternary alloy ishighly wettable with a carbonaceous material and provide a stableemission of ions which continues even about 100 hr after initiation ofthe emission. A similar effect could be attained by replacing Si with Sbor Ge, or by using at least two of these elements in combination.

It is preferred that the amount of Sb, Si and Ge to be added be at least5 atomic %. When the amount is less than the above range, anysatisfactory improvement in the wettability with a carbonaceous materialcan be attained. On the other hand, when Sb, Si or Ge is added in anexcessive amount and amounts to the major part of the alloy, the amountof ionic current of intended B ions is decreased. Therefore, it ispreferred that the amount to be added be at most 50 atomic % based onthe total.

Such an effect can also be attained with respect to Pd-B alloy and Ag-Balloy besides Pt-B alloy.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustrative view of a mass spectrum obtained in an exampleof the present invention;

FIG. 2 a schematic cross-sectional view of a liquid metal ion sourceused in an example of the present invention; and

FIG. 3 an illustrative view of a mass spectrum obtained in anotherexample of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The examples of the present invention will now be described in moredetail with reference to the attached drawings.

EXAMPLE 1

FIG. 2 shows a constitution of a liquid metal ion source according tothe present invention. A source material 5 for this ion source is meltedby means of electroheating. An emitter 1 is connected to a support 2which is connected to an insulating material 14. A reservoir 3 whichserves also as an electroheating heater for melting the source material5 is fixed at its both ends to electric current lead-in terminals 4,4'.The reservoir 3 at its center has a circular hole 6 through which theemitter 1 wetted with the source material 5 in a molten state is passed.FIG. 2 shows the emitter 1 wetted with the molten source material 5,which is protruded from the circular hole 6 provided at the reservoir 3.Numeral 7 designates an extracting electrode. The application of anelectric field of a several kV between the electrode 7 and the emitter 1brings about a downward extraction of an ion beam 8 from the emitter 1through a through-hole 9 formed in the electrode 7. In the presentexample, the emitter is 0.3 mm in diameter and made of tungsten (W) andhas a tip sharpened by means of electropolishing to such an extent thatthe radius of curvature is several μm. The reservoir 3 which serves alsoas a heater is made of a molybdenum (Mo) plate having a thickness of 0.1mm, and the recess provided at the center thereof is worked so that itcan store the source material 5 in an amount of several mm³. Thediameter of the circular hole 6 provided at the center of the reservoir3 is about 1 mm.

In FIG. 2, numeral 10 designates a heating power source for the sourcematerial 5, numeral 11 an ion extracting power source, numeral 12 an ionaccelerating power source and numeral 13 a vacuum container.

In the present example, Pt₆₄ As₂₄.5 Sb₁₁.5 was used as the sourcematerial 5. The melting point of the source material 5 is about 600° C.The source material 5 was put on the heater 3 which served also as areservoir and heated to about 700° C. When the ion source was thenoperated, an ion beam 8 was stably emitted. In order to subject theemitted ions to mass spectrometry, the ion source was mounted on a massspectrometer (not shown) equipped with a magnetic sector. A typicalexample of the mass spectra thus obtained is shown in FIG. 1. Themass/electric charge ratio, i.e., m/e is plotted as abscissa and the ionintensity (arbitrary unit) is plotted as ordinate. The ion extractingvoltage is 5.7 kV and the total emitted ion current is 20 μA.

From the spectrum, it can be seen that the present ion source emits ionssuch as As⁺, As²⁺, Pt⁺, Pt²⁺, Sb⁺ or Sb²⁺ and that with respect to Asions the ion intensity of As²⁺ is higher than that of As⁺.

A large amount of emission of As²⁺ brings about the following effect.For example, it is expected that the ion source of the present inventioncan be applied to an ion implantation process for a semiconductor. As⁺which has been accelerated by means of a certain accelerating voltage V(kV) is implanted into a semiconductor substrate with an energy of V(keV). On the other hand, since As²⁺ has a doubled energy, i.e., 2V(keV), As²⁺ is implanted into the substrate more deeply than with As⁺.Specifically, when As⁺ and As²⁺ which have been accelerated with V of100 (kV) are implanted into a Si substrate, the penetrations (range) ofAs⁺ and As²⁺ are about 0.06 μm and 0.11 μm, respectively, i.e., As²⁺ islarger in range. Therefore, the implantation of As⁺ and As²⁺ by properlymaking use of them enables these ions to be implanted to differentdepths at the same accelerating voltage.

The effect attained by the present example is that the addition of Sb tothe Pt-As alloy suppresses the rise of the melting point of the sourcematerial, which effectively suppresses selective evaporation of As ascompared with the case where conventional Pt-As binary alloy is used asthe source material. Therefore, the present ion source canadvantageously emit a desired As ions for a long period of time.Specifically, the present ion source continuously emitted As ions even100 hr in total after initiation of ion emission without causing anysignificant change in both the ionic currents of As⁺ and As²⁺.

Further, the present example is also characterized by emission of Sbions. Sb is also an element belonging to Group V and serves as a dopantfor a Si substrate. Therefore, two kinds of n-type dopants which aredifferent in mass from each other, i.e., As and Sb, can be emitted fromthe present ion source, and with respect to the both ions the divalentions are emitted in larger amount.

With respect to the above-mentioned prevention of selective evaporationof As by addition of the third element to a Pt-As alloy which is a noblealloy, the same effect can be attained by using Si or Ge instead of Sbused in the present example and at least two elements selected fromamong Sb, Si and Ge. Specifically, Pt₆₄ As₂₅ Si₁₁, Pt₅₈ As₂₂ Sb₁₀ Si₁₀,Pt₆₄ As₂₅ Ge₁₁, etc. can be used as the source material.

EXAMPLE 2

The same liquid metal source as the one used in Example 1 was used inthis example, except that a source material having a composition of theformula Pt₆₈ P₁₇ Sb₁₅ and a melting point of about 600° C. was used asthe source material 5 in this Example 2 instead of the source material 5as used in Example 1.

This ion source was operated at about 700° C., and it was confirmed thata stable ion was emitted. The results of the mass spectrometry of theemitted ions are shown in FIG. 3. The total emitted ionic current I_(T)is 20 μA. The mass spectra show peaks of P⁺, P²⁺, Sb⁺, Sb²⁺, Pt⁺, Pt²⁺and other peaks of molecular ions having a small intensity. The ionsource of the present example continued to stably emit ions at arelatively low melting point (800° C. or below) as in Example 1, and nosignificant change in mass spectrum pattern was observed even 150 hr intotal after initiation of ion emission. This suggests that nosignificant selective evaporation of P from the molten source materialtook place. The reason for this is believed to reside in that theincorporation of Sb suppressed the rise of the melting point.

The service life was about 200 to 300 hr, i.e., about 10 to 15 timeslonger than that of the Pt-P ion source.

It can particularly be seen from FIG. 3 that the ion intensity of P²⁺ ishigher than that of P⁺.

When the source material is a Cu-P binary alloy, the ratio of intensityof P²⁺ to that of P⁺ emitted, i.e., P²⁺ /P⁺ ratio, is extremely small.Particularly, with respect to emission of P²⁺ there is no description inthe prior art. However, in the present example, the intensity ratio P²⁺/P⁺ is about 1 to 3, i.e., P²⁺ is emitted in an amount larger than thatof P⁺. It is noted in this connection that the intensity ratio P²⁺ /P⁺depends on the total emitted ionic current I_(T) and that the ratio ismaximum when I_(T) is about 10 μA.

Since the ion intensity of P²⁺ is higher, the use of the P²⁺ ion beamfor ion implantation leads to an advantage that P²⁺ can be implantedmore deeply than the monovalent ion.

Another effect attained by the present example is that the resolution ofthe mass spectrometer provided after the extracting electrode 7 in orderto obtain a single beam of P²⁺ may be small, because there exists nopeak of other element ion around P²⁺ peak, as can be seen from the massspectrum shown in FIG. 3. In the present example, in order to obtain aP²⁺ ion beam, the mass resolution may be 10 or less. On the other hand,with respect to the Cu-P ion source, in order to obtain an ion beam ofP⁺ which is the maximum peak in the P ions, it is necessary to separate³¹ P⁺ from ⁶³ Cu²⁺, which requires the use of a mass spectrometer havinga mass resolution of 62. Therefore, the resolution required forseparating P²⁺ from the Pt-P-Sb ion source may be 1/6.

With respect to the above-mentioned prevention of selective evaporationof P by addition of the third element to a Pt-P alloy which is a noblealloy, the same effect could be attained by replacing Sb in the aboveternary alloy Pt-PSb with Si or Ge or Si and Ge, Si and Sb, Sb and GE,or Si and Sb and Ge, i.e., at least one element selected from among Sb,Si and Ge. Specifically, Pt₆₄ P₁₆ Si₂₀, Pt₆₇ P₁₆.5 Ge₁₆.5, Pt₆₄ P₁₆ Sb₁₀Si₁₀, Pt₆₀ P₁₅ Ge₁₂ Si₁₃, etc. can be used as the source material.

EXAMPLE 3

Ag, As and Ge powders were mixed so that the atomic composition is Ag₆₀As₃₂ Ge₈. The mixture was molded with a compression molding machine intoa cylindrical shape having a diameter of 5 mm and a height of 10 mm. Aglass-made ampule was charged with the resulting molding and then sealedwith Ar under pressure. The ampule was placed in an electric oven tomelt the molding of Ag-As-Ge. The sealing with Ar under pressure is forprevention of evaporation of As during melting.

The melting points of Ag and As are lowered to about 540° C. in the formof a composition Ag₇₅ As₂₅. Since Ag and As both exhibit a high vaporpressure when melted in the form of a simple element, they cannot beused as the source material. Further, with respect to the binary alloyAg-As, the melting point thereof rises several hours after initiation ofemission, which makes it difficult to stably extract ions. This is alsoattributable to the fact that Ag and As evaporate to change thecomposition of Ag-As. However, the incorporation of Ge in the Ag-Asbinary alloy suppressed the rise of the melting point and maintained thealloy in a molten state at substantially the same temperature for a longperiod of time, which led to continuous feeding of the liquid metal tothe tip of the emitter. As a result, the ion source continued to stablyemit a desired As ions even 100 hr in total after initiation of emissionof ions.

The above-mentioned effect could also be attained by replacing Ge in theabove ternary alloy Ag-As-Ge with Sb or Si or Sb and Si, Si and Ge, andSb and Ge. Specific examples of such compositions include Ag₆₀ As₂₄Sb₁₆, Ag₆₀ As₂₅ Si₁₅, Ag₅₅ As₂₁ Sb₁₄ Ge₁₀, Ag₅₄ As₂₃ Si₁₃ Sb₁₀, Ag₅₄As₂₃ Si₁₃ Ge₁₀ and Ag₅₀ As₂₁ Si₁₂ Sb₉ Ge₈. These alloys exhibit nosignificant change in melting point, and the effect attained by addingSi, Sb and Ge to the noble alloy Ag-As was observed.

In the present example, it is noted that the use of a mass spectrometerhaving a resolution of at least 75 is required for separation ofmonovalent ions of ⁷⁴ Ge and ⁷⁶ Ge which are isotopes of Ge frommonovalent ions of ⁷⁵ As and separation of divalent ions of ⁷⁴ Ge and ⁷⁶Ge from divalent ions of ⁷⁵ As.

EXAMPLE 4

In the present example, a Pt-B-Si ternary alloy was used as a sourcematerial. As Pt-B eutectic alloy (Pt₆₀ B₄₀ : melting point of about 830°C.) powder and a Pt-Si eutectic alloy (Pt₇₇ -Si₂₃ : melting point ofabout 830° C.) powder were mixed. The mixture was molded with acompression molding machine into a cylindrical shape in the same manneras in Example 3. The molding was melted in an electrical oven to obtaina Pt₆₅ B₂₈ Si₇ ternary alloy.

As mentioned above, since B and alloys containing it react in a moltenstate with other metals, tungsten (W) and molybdenum (Mo) which haveconventionally been used for liquid metal ion sources cannot be used asmaterials for the emitter and the reservoir. In some cases, acarbonaceous material has been used to solve this problem. However,metals wettable with the carbonaceous material are limited in kind, andPt, Pd, etc. are hardly wetted with the carbonaceous material.Therefore, difficulties are encountered in constructing a liquid metalion source using a Pt-B alloy as a source material and a carbonaceousmaterial as a material for the emitter and the reservoir. However, theaddition of Si to the Pt-B alloy improved wettability thereof with thecarbonaceous material. Specifically, when an emitter made of tungstencarbide (WC) and a reservoir, which served also as a heater, made ofcarbon (C) was used, ions were stably emitted. As a result of massanalysis, it was found that the ionic current of the desired B⁺ ionsamounted to about 20% of the ionic current which have reached thesample. The ionic current of B⁺ hardly changed even 100 hr afterinitiation of emission of ions, which suggested that ions was stablyemitted from the ion source.

With respect to the above-mentioned improvement in wettability by addingthe third or fourth element to a B containing alloy which is difficultywetted with a carbonaceous material, the same effect as the one inExample 5 could be attained by incorporating a Pd-B alloy and an Ag-Balloy with Si or Sb or Ge, or at least two elements out of the abovethree element in combination. Of course, the same effect can be attainedby incorporating a Pt-B alloy with Sb or Ge, or at least two elements incombination selected from among Si, Sb and Ge. Specific examples of suchcompositions include Pd₅₈ B₂₂ Sb₂₀, Pd₆₆ B₂₄ Ge₁₀, Pt₅₄ B₃₆ Ge₁₀, Ag₆₇B₂₃ Sd₁₀, Ag₆₇ B₂₃ Si₁₀, Ag₆₇ B₂₃ Ge₅ Si₅ and Pt₅₃ B₂₅ Sb₇ Ge₅.

EXAMPLE 5

In the present example, a Pt-B-P-Sb quaternary alloy was used as thesource material. Specifically, Pt was used as a matrix metal, Sb as anelement for suppression of the rise of the melting point and twoelements B and P as desired elements. This ion source is for emittingtwo kinds of elements, i.e., n-type (P) and p-type (B).

Conventionally, Pt-B and Pt-P liquid metals had a problem that theycould not be wetted with a carbonaceous material, and the Pt-P alloyalso had a problem with selective evaporation. Therefore, difficultieswere encountered in developing an ion source which used Pt as a matrixmaterial and emitted both of B and P ions from one ion source. Althoughit is possible to emit B and P ions while sacrificing the service lifeof the ion source by using a metallic material such as tungsten for theemitter and the reservoir and further lowering the B content to avoidthe reaction with the metal, such a method is disadvantageous from apractical point of view.

However, in the present example, the above problem was solved by addingSb to the Pt-B-P alloy. Specific Example of such a composition includesPt₆₄ B₂₃ P₇ Sb₆.

The present example brings about the following effects. Specifically, itis needless to say that ions of n-type (P and Sb) and p-type (B) couldbe emitted from one ion source. The addition of Sb reduced selectiveevaporation of P, and there was caused no significant changes in themelting point of the source material and in emitted ion intensity evenabout 100 hr in total after initiation of emission of ions. Further, theaddition of Sb improved the wettability with a carbonaceous material andenabled the use of carbides such as tungsten or titanium carbide asmaterials for the emitter and the reservoir. Since these materials donot react with B, the service life of the ion source can be improvedeven when a source material containing B in an increased amount is used.

With respect to the above-mentioned suppression of the rise of themelting point accompanying evaporation of an element having a high vaporpressure and the improvement in wettability with a carbonaceousmaterial, the same effect can be attained by using Si and Ge instead ofSb in the above-mentioned quaternary alloy.

Further, the same effect can be attained by replacing P in theabove-mentioned quaternary alloy with As.

Besides Examples 1 to 5, there were conducted some experiments onemission of ions with respect to the source material containing variedamounts of P, As and B. As a result, it was found that in order toincrease the ionic current of desired P, As or B, it was preferred toincrease the amount to be added. However, too large an amount of P, Asor B causes P and As to be easily evaporated during melting of a sourcematerial as well as causes the rise of the melting point due to anincrease in the amount of B. Selective evaporation of P and As bringsabout changes in composition of the source material, which leads to therise of the melting point. From this, there arise problems such asnecessity of an increase in the current for heating a heater in order tomelt these metals and a decrease in the ionic current of P or As due toevaporation of P and As. Therefore, in order to reduce selectiveevaporation of P and As, to keep a relatively low melting point (600° to1000° C.) for a long period of time and to attain a stable ionic currentof a desired element for a long period of time, it is preferred that theamount of P, As or B to be added be at most 50 atomic % based on thetotal amount of the composition.

INDUSTRIAL APPLICABILITY

As is apparent from the foregoing descriptions, the present inventioncan provide a liquid metal ion source capable of stably extracting ionsof at least one element selected from among phosphorus (P), arsenic (As)and boron (B) for a long period of time. The liquid metal ion source ofthe present invention can be used as an ion source for an ion implanter,a micro-zone secondary ion mass spectrometer, a micro-zone depositionapparatus or the like.

We claim:
 1. In a liquid metal ion source comprising a reservoir whichcontains a source material in a molten state, an emitter which has beenarranged so that said molten source material fed from said reservoir isemitted in the form of ions from the tip thereof and an extractingelectrode which serves to extract ions from the tip of said emitter, theimprovement wherein said source material is an alloy having acomposition represented by the formula L_(X) R_(Y) M_(A) wherein X, Yand A each stands for atomic percentage; L at least one element selectedfrom a group consisting of Pt, Pd and Ag; R at least one elementselected from a group consisting of As, P and B; M at least one elementselected from a group consisting of Ge, Si and Sb; 5<A<50; 40<X<70; andX+Y+A=100.
 2. A liquid metal ion source according to claim 1, whereinsaid source material is an alloy having a composition represented bysaid formula wherein L stands for at least one element selected from agroup consisting of Pt and Pd and R at least one element selected from agroup consisting of P and As.